In your g-code, M6 T1 will change it to tool 1. If you want to use Length Offsets, then use G43. See thw manual on how to use it.
That's a pretty old version of Mach3, and I'm not sure if the following still holds true for your version. If you want to change tools in a program, you call M6 Tx. In Mach's cofiguration screen (Config>General Config), you can set a few different options for what Mach3 will do when it sees an M6.